Black Discharge in the middle of my cycle?

I had a little black discharge in the middle of my cycle (i.e. during ovulation). By a little I mean maybe a little more than the size of a fingernail....

Other maybe related symptoms: back pain (near kidneys), boil or cyst on the outside of my vagina and strong smell (always there though even after a good shower) ...

I just went for an ultrasound today of my kidneys so I'll get the results in a few days...

Thanks for your answers!

I hope you understand why I posted this anonymously....